I'm obsessed with doors.
Okay, not really. But I do find I mention them a lot when I'm writing, sometimes so often it gets ridiculous and I have to start cutting. The thing is, doors are just so useful.
Doors and aliens: the shape and mechanism of a door will tell you a lot about the physiology of a species, even when they are absent. If a door gets narrower than a certain width, human beings will shift their shoulders sideways to go through it even though it may be wide enough to admit them without the shift. So what about aliens? How do they go about entering a space? Do they slither and need just a tiny hole, or are they enormous? Do they manipulate a handle to open the door? Does the door have hinges or does it slide? Does the door lock? These questions will illuminate local technology. Another question relevant here is how to know who is on the other side of the door. Is there a peephole to allow the apartment-dweller to see who is outside? Or will the individual on the inside use hearing or smell to determine who the visitor is?
Doors and power structures: who gets to sit behind a door? And who gets to sit, or stand, in front of it? These are things that can reveal a lot about the status structure of your society. If the people behind the door are knowledge keepers, then maybe a great deal of value is placed on certain types of knowledge, and this is then kept secret from others (I'm thinking of bureaucrats, but also religious knowledge and others can fall into this category).
Doors and manners: there are rules about how doors must be treated. In the US, an open door is seen as friendly. In Germany, it's seen as sloppy. The French see open doors and worry about draughts in the house. Whether the door is kept open or closed does not always say the same thing about the person inside. Maybe it says "do not disturb," but maybe it says "I maintain appropriate aloofness but you may approach me." Are people expected to knock to gain admission? And how should they open the door? In my house, where I've often got kids or stuff of various kinds in hand, I open the door any way I can. In Japan, opening a door with anything but your hands is bad manners (don't use your feet or your posterior!). In Japanese restaurants, the servers will put their trays down and remain kneeling to open the sliding door with one hand placed close to the floor.
Doors and point of view: I think this is why I use doors so much. If you're doing close limited point of view, it really helps to differentiate between describing actions from the inside of the POV character, or from the outside for other characters.
For the POV character, a door can mean a lot of different things. It can mean imprisonment. It can mean fear or resentment, that someone in particular might come in. It can mean safety. It can be the last line of protection. A character can approach a door with hesitation, paranoia, eagerness, excitement or apprehension. Or irritation, as Arthur Dent did with the sighing doors of the Heart of Gold spaceship in The Hitchhiker's Guide to the Galaxy.
For the non-POV characters, a door can help you show their mental states without actually having to describe the emotion. How does the character approach the door? Quickly? With impatient movement, or with reluctance? Does he or she approach it face-first or back-first? What body part does your character use to knock, or to open the door? How does he or she grasp the handle - with white knuckles, or gingerly, or firmly? What quality of sound or air movement does the opening of the door create? All of these things can broadcast the inner states of a non-POV character, and the writer can choose whether to complete the extrapolation of the associated mental state or not. This means that if you describe how the person handles the door, but without using any direct descriptions of the assumed mental state, the reader can draw two conclusions: first, the reader can extrapolate the mental state of the person who went through the door, and second, the reader can deduce that the point of view character didn't draw the same conclusions, i.e. that he or she may have been unaware of the other person's mental state. And that's one way to show things to your reader while hiding them from your POV character in tight internal/limited point of view.
You gotta love doors.